The invention relates to an apparatus for determining the filling level of a filling material in a container having a signal production unit which produces measurement signals, having at least one antenna which transmits the measurement signals in the direction of the surface of the filling material and which receives the measurement signals reflected on the surface of the filling material, and having a control/evaluation unit which uses the delay time of the measurement signals to determine the filling level of the filling material in the container. The measurement signals are preferably ultrasound signals or microwave signals.
Delay time methods make use of the physical law according to which the propagation distance is equal to the product of the delay time and the rate of propagation. In the case of filling level measurement, the propagation distance corresponds to twice the distance between the antenna and the surface of the filing material. The useful echo signal, that is to say the signal reflected on the surface of the filling material, and its delay time are determined using the so-called echo function and the digitized envelope curve, in which case the envelope curve models the amplitudes of the echo signals as a function of the distance xe2x80x9cantennaxe2x80x94surface of the filling materialxe2x80x9d. The filling level is then itself obtained from the difference between the known distance between the antenna and the base of the container and the distance between the surface of the filling material and the antenna, as determined by the measurement.
All known methods can be sued which allow relatively short distances to be determined by means of reflected measurement signals. If the measurement signals are microwaves, then both pulsed radar and frequency-modulated continuous wave radar (FMCW radar) can be used. Microwave measurement devices which use pulsed radar are marketed by the applicant, for example under the name xe2x80x9cMICROPILOTxe2x80x9d. An equipment type which operates with ultrasound signals is available from the applicant, for example, under the name xe2x80x9cPROSONICxe2x80x9d.
In both the known ultrasound measurement devices and in microwave measurement devices, the antennas via which the measurement signals are transmitted in the direction of the surface of the filling material, and via which the measurement signals reflected on the surface of the filling material are received must be located in the region of the lid of the container. This arrangement is necessary in order that the measurement signals strike the surface of the filling material essentially at right angles. One antenna is preferably positioned in a connecting stub which is already present in the lid of the container. In situations in which there is no opening, the opening must be provided specifically for fitting the antenna. In the simplest case, the measurement apparatus is mounted in the connecting stub opening in the lid via a flange. Installation and maintenance of a measurement device in the lid region of a container has been found to be tedious and difficult, particularly when no opening is provided there and an opening must be produced in advance. The installation and maintenance of the measurement device in containers having large geometric dimensionsxe2x80x94that is to say the situation which is normal in industrial process technology and instrumentationxe2x80x94are particularly problematic.
However, in many cases, there are also openings in the sidewall of the container in which the filling material is stored. These openings may be used, for example, to accommodate a so-called bypass, that is to say a piece of pipe, which is arranged parallel to the outer wall of the container. However, they may also be provided in order to fit a differential pressure sensor. For both variants, an opening must be provided in each case both in the lower region and in the upper region of the sidewall of the container. Furthermore, the already existing opening in the sidewall may be a retaining opening for a pressure or temperature sensor, or for a limit switch for determining and/or monitoring the maximum filling level of a filling material in the container. Whatever the original purpose of this opening, if it is located in the upper region of the sidewall of the container, then it can be used in conjunction with the apparatus according to the invention.
The invention is based on the object of proposing an apparatus which allows a filling level measurement device, which operates on the delay-time principle, to be mounted on a container in a cost-effective and simple manner.
The object is achieved in that an opening is provided in the upper region of one sidewall of the container, and in that the at least one antenna is positioned in this opening with the antenna being arranged and configured such that the measurement signals are emitted essentially in the direction of the filling material and such that the measurement signals reflected on the surface of the filling material are received by the antenna. The transmission unit and the reception unit can, of course, also be designed as separate units, in which case it is entirely possible to arrange both antennas in an apparatus which is designed as an integral unit.
As has already been mentioned above, openings in the upper region of the sidewall of the container are, on the one hand, preferably used for installation of the filling level measurement device where, on the other hand, the openings already exist there. Both measures allow the installation and maintenance of the filling level measurement device to be considerably simplified, of course. In many cases, the customer would also like to replace the existing differential pressure instrumentation by instrumentation which operates using electromagnetic measurement signals.
The replacement of a differential-pressure sensor by a measurement device which operates with freely transmitted measurement signals at the same time offers further advantages:
Reduction in the installation complexity: in principle, two variables have to be detected for differential-pressure measurement, namely the static pressure of a liquid and, for example, the pressure of a gas cushion. At least two openings must be provided in the outer wall of the container in order to detect the two variables. Furthermore, a pipeline is required to connect the two measurement points. Such pipelines result in a risk of blockage, particularly when measuring the pressure in viscous liquid filling materials.
Risk of the filling material flowing out: openings in the lower region of the container always represent a risk, of course, particularly when toxic filling materials are being stored in the container.
Labor cost for maintenance and replacement of a differential-pressure sensor: in order to replace a device which is arranged in the lower region of the container, it is in principle necessary to empty the container in advance.
As already stated, in the context of the apparatus according to the invention, it is possible in a very simple manner to replace a differential-pressure sensor mounted in the sidewall of the container by a microwave or ultrasound sensor.
According to one advantageous development of the apparatus according to the invention, the antenna is essentially an elongated element, whose external dimensions in the longitudinal direction are greater, and in the transverse direction are less, than the internal dimensions of the opening. This configuration makes it possible to fit the antenna from the outside through the sidewall into the interior of the container, and to adjust it such that the measurement signals are emitted essentially in the direction of the surface of the filling material. The antenna is most easily adjusted by appropriate rotation of the elongated element about the longitudinal axis. The antenna may be, for example, a leaky waveguide, a ridge waveguide or a Yagi antenna. It is also possible to configure the antenna as a horn radiating element with a symmetrical or asymmetric horn, and with the horn preferably being integrated in the elongated element.
Alternatively, a separate rod antenna or a separate horn antenna may also be provided as the antenna. The rod antenna or the horn antenna is then preferably arranged such that it can be folded in the region of the end face of the elongated element. In order to allow the antenna to be fitted into the interior of the container through the opening in the container wall, the rod or horn antenna is oriented in the direction of the longitudinal axis of the elongated element, with the antenna in this position being dimensioned such that it can be pushed through the opening from the outside. If the elongated element and, in particular the antenna are arranged in the interior of the container, the rod or horn antenna is pivoted through 90xc2x0, so that it can now transmit the measurement signals in the direction of the surface of the filling material, and can receive the measurement signals reflected on the surface.
One advantageous embodiment provides an additional sensor, which determines at least one process variable in the container. This additional sensor is connected to the antenna, which determines the filling level on the basis of the delay time of measurement signals. The additional sensor is preferably mounted in the direction of the longitudinal axis of the antenna. The additional sensor may be, for example, a vibration detector, that is to say a limit switch for determining and/or monitoring the filling level in the container, a pressure sensor or a temperature sensor. One advantageous development of the apparatus according to the invention proposes that two antennas be arranged in or on the elongated element, which transmit measurement signals at different frequencies in the direction of the surface of the filling material, and which receive the measurement signals reflected on the surface of the filling material. Furthermorexe2x80x94as already described at a previous pointxe2x80x94the invention provides for two antennas to be arranged in and/or on the elongated element, with the one antenna being used as a transmission unit and the other being used as a reception unit for measurement signals at a predetermined frequency. In the latter case, this is thus a filling level sensor with a separate transmission and reception unit.
One particularly advantageous development of the apparatus according to the invention proposes that at least the additional sensor be a sensor which complies with a specified safety standard. The following safety standards may, for example, be satisfied by the sensor: German Water Budget Act, Order on Combustible Liquids, Safety Integrity Level (SIL). The advantage of such an apparatus for the user is that lower insurance premiums are incurred when using a sensor which complies with at least one safety standard, and this can considerably reduce the operating costs.
One advantageous development of the apparatus according to the invention provides for a metallic shield to be located in the antenna region which is arranged in the opening in the sidewall of the container, or for a metallic shield to be located in an antenna region which is arranged in the immediate vicinity of the opening in the sidewall of the container. This refinement makes it possible to effectively eliminate interference signals which are caused by reflections in the opening in the container wall (the so-called connecting stub ringing), on the container wall or on adjacent fittings in the interior of the container. The shield may, for example, be a metal sleeve.
In order to protect the antenna and/or the sensor of the apparatus according to the invention against the influence of a corrosive filling material, one refinement of the apparatus according to the invention proposes that the additional sensor and/or the antennas be provided with a protection layer, in particular with a dielectric protection layer, at least in the region which projects into the interior of the container. This refinement is a major advantage, particularly if the antennas and/or the additional sensor come/comes into direct contact with the filling material. Protection layers which may be used in conjunction with the present invention are already known, for example from EP 0 669 673 B1.
In order to simplify maintenance and installation, one advantageous embodiment of the apparatus according to the invention provides an outer housing which is mounted in the opening in the upper region of the sidewall of the container and provides that the antenna, or the antenna with the additional sensor, be positioned in the outer housing. The outer housing is firmly connected to the container, for example via a flange. The outer housing is preferably composed of a dielectric material. However, it is also possible to manufacture the outer housing from a conductive material, winch has a cutout on the side facing the filling material. An insert composed of a dielectric material is provided in this cutout, which acts as an outlet or an inlet opening for the measurement signals.
In order to ensure optimum transmission and optimum reception of the measurement signals, the antenna is arranged in the outer housing such that it can rotate or can pivot about its longitudinal axis. It can thus be adjusted with respect to the cutout described above, without any problems. Adjustment is necessary, in particular, if the pattern of the holes in the customer""s flange is unknown. Transmission in the direction of the filling material is, however, ensured only if the transmission direction is variable and can be adjusted relative to the pattern of the holes in the flange.
According to one advantageous refinement of the apparatus according to the invention, the measurement signals are passed to the antenna from the signal production unit via a conductive element. The conductive element is, for example, a coaxial cable, a waveguide or a hollow conductor. It is also possible, of course, for the measurement signals to be fed directly into the antenna via a board with a microstripline.